1. Field of the Invention
The present invention relates generally to a time synchronizing apparatus for a mobile WiMAX analyzer and, more particularly, to a time synchronizing apparatus for a mobile WiMAX analyzer, which is provided with some of the functions performed by individual radio access stations, so that time synchronization, which is essentially required between mobile WiMAX analyzers capable of carrying out handover tests for mobile WiMAX terminals, can be maintained.
2. Description of the Related Art
In a typical mobile communication system, the communication protocols between radio access stations may be classified into a synchronous type and an asynchronous type. Here, a synchronous scheme refers to a scheme for performing time synchronization between radio access stations using the common clock of a Global Positioning System (GPS), and an common clock of a Global Positioning System (GPS), and an asynchronous scheme refers to a scheme for not performing synchronization between the radio access stations. Unlike the synchronous scheme, the asynchronous scheme has an advantage in that flexibility for the installation of radio access stations is assured because it is not necessary to receive a GPS signal, but it has a disadvantage in that it takes a lot of time for a mobile communication terminal to detect a radio access station or achieve synchronization. For the synchronous scheme, existing IS-95-based cellular, PCS and Code Division Multiple Access (CDMA) 2000 communication methods are used. For the asynchronous scheme, a Wideband Code Division Multiple Access (W-CDMA) communication method, which was proposed in Japan and Europe, is used.
The mobile communication radio access stations, described above, are provided with respective atomic clocks, and synchronization between a calling party's mobile communication terminal and a called party's mobile communication terminal is performed using signals generated by the respective atomic clocks of the mobile communication terminals. In the case where a mobile communication terminal moves from a radio access station region to another radio access station region, the atomic clocks, which are provided in the respective radio access stations, must keep the same time so that high-quality crosstalk-free communication can be performed. For example, the atomic clocks must keep time to about one-millionth of a second, so that the mobile communication terminals operate normally. For this purpose, a signal that is generated by an atomic clock provided in the GPS is used.
Meanwhile, currently, schemes for wirelessly accessing the Internet are classified into a scheme for making access via a mobile telephone network based on a Wireless Application Protocol (WAP) or a Wireless Internet Platform for Interoperability (WIPI) platform, and a scheme of making access via a Public Wireless Local Area Network (LAN) and an access point. However, the former scheme has a fundamental limitation in that it cannot be widely used as an Internet access means due to the screen size, limitations of the input interface, a billing system based on usage-based charging, and the like. Furthermore, the latter scheme also has fundamental problems in that it can be used only within a radius of several tens of meters around the access point, and in that mobility is poor. In order to overcome these problems, a mobile WiBro or WiMax system has been proposed to provide a wireless Internet service that enables high-speed Internet access in a stationary state or in a low-speed moving state with Asymmetric Digital Subscriber Line (ADSL)-class quality and cost.
In such a mobile WiMAX system, Time Division Duplex (TDD) is employed as a duplex scheme, and Orthogonal Frequency Division Multiple Access (OFDMA) is employed as a multiple access scheme. In a TDD-based system, a signal transmitted to a mobile WiMAX terminal by a radio access station and a signal transmitted to the radio access station by the mobile WiMAX terminal shares the same frequency band but uses different time slots, so that, when the radio access station is not accurately synchronized in time, the performance of the system can be degraded due to collisions between the uplink and downlink signals of the radio access station and the mobile WiMAX terminal. Furthermore, the inaccuracy of a synchronization signal causes symbol timing offset and frequency offset in a signal received by the mobile WiMAX terminal. In particular, this becomes a main factor that degrades the performance of a system that uses an OFDM scheme.
Furthermore, in an environment including a plurality of cells, as in the mobile WiMAX system, each radio access station must support the handover of the mobile WiMAX terminal. Here, the term ‘handover’ refers to a process in which a mobile WiMAX terminal moves from a radio access station that provides a wireless interface to another radio access station. The handover is performed when it is necessary to change the radio access station to which the mobile WiMAX terminal is connected in order to provide a higher-quality signal, according to the degrees of signal attenuation and interference occurring when the mobile WiMAX terminal moves, or is performed when the mobile WiMAX terminal can receive higher Quality of Service (QoS) from another radio access station.
Accordingly, in the case of mobile WiMAX analyzers, each of which is provided with a function such as the above-described handover function, which is performed by each radio access station, and is capable of carrying out handover tests for mobile WiMAX terminals, it is essential that time synchronization be maintained between the analyzers. However, conventionally, an analyzer that is provided with the above-described function has not been proposed, and thus there is a problem in that reliable handover tests cannot be performed.